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An Assessment of the High-Gain Streckheisen STS2 Seismometer for Routine Earthquake Monitoring in the US

An Assessment of the High-Gain Streckheisen STS2 Seismometer for Routine Earthquake Monitoring in the US. ISSUE: Is the high-gain STS2 too sensitive to record on-scale significant earthquakes within the continental US.

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An Assessment of the High-Gain Streckheisen STS2 Seismometer for Routine Earthquake Monitoring in the US

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  1. An Assessment of the High-Gain Streckheisen STS2 Seismometer for Routine Earthquake Monitoring in the US ISSUE: Is the high-gain STS2 too sensitive to record on-scale significant earthquakes within the continental US.

  2. Methods. Three different methods were used to examine the levels of input ground velocity expected during routine earthquake monitoring within the continental US. • 1. Computing PGV expected at each ANSS backbone station from the USGS hazard maps (Frankel, 2000). • 2. Modeling seismic wave amplitude (Brune, 1970) • 3. Analysis of the Parkfield M6 Earthquake, September 2004

  3. Standard ANSS Backbone Station STS2 Sensitivity: high-gain STS2 = 20,000 volts/meter/sec standard-gain STS2 = 1500 volts/meter/sec Q330 channel sensitivity = 4.19e5 counts/volt Q330 clip level = 8.38e6 = 8388608 counts Input ground velocity required to clip Q330 high-gain STS2 = 0.001 m/sec standard-gain STS2 = 0.013 m/sec (factor of 13 larger)

  4. Method I: PGV computed from 1Hz Spectral Acceleration Map. • Use hazard map to calculate the expected PGV at each ANSS backbone station. Procedure: 1. For each ANSS station determine %g from the 1 Hz spectral acceleration map and convert to g (g = 0.01* %g) where 1g=980cm/s/s. • 2. Compute PGV in m/sec after (Newmark and Hall, 1982) • PGV = 594.8 * g / (2*p). • 3. Compare PGV to clipping velocity of standard and high-gain recording systems. 1 Hz spectral acceleration 10% probability of exceedance in 30 years (Frankel et al., 2000)

  5. Method I: Results Most stations clipped by expected local ground motion except regions far from active earthquakes. All stations clipped by expected local ground motion. USGS 10% probability of exceedance in 30 years 1Hz spectral acceleration (Frankel et al., 2000)

  6. Method II: Amplitude Modeling using Brune (1970, 1971) • 1. For each Mw (5-7) determine Mo from: Mw = 0.667 log(Mo) – 10.7 (Kanimori, 1977). • 2. For each Mo determine fault dimension from: • Mo = 2.29r3 dyne-cm (Brune 1970, 1971) • where  = 1.0x108 effective stress. • 3. For each fault dimension, determine source corner frequency from: • fault dimension in cm (Brune 1970, 1971). • where  = shear wave velocity = 3.5km/s • fc = source corner frequency • 4. Find angular frequency (fm), by bisection, where Brune source spectrum is a maximum given ANSS short-period detection filters (0.5-12Hz). • 5. For each source-receiver distance (from 25-1100 km), calculate Shear-wave PGV amplitude (As) in m/sec from: • (Brune 1970, 1971). • where  = rigidity • fc = source corner frequency • fm = effective frequency where amplitude is maximum •  = shear-wave velocity (3.5km/s) • = source-receiver distance. • 5. Apply attenuation to Shear-wave amplitude (As). • where  = geometric spreading (0.5) • Q = Quality factor. • Northern California (Erickson et al. 2004). • Q = 105fm 0.67

  7. 6. Multiply the modeled amplitude by the total sensitivity of the high-gain recording system (8.38e9 counts/m/s) and plot corrected PGV as a function of epicentral distance.

  8. Method III: M6.0 Parkfield Earthquake, September 2004 Lg COR 1003km STS1 Q380 COR MOD MOD 667km STS1 QX-80 WDC HOPS WDC 558km STS2-SG Q380 Parkfield M6 HOPS 421km STS1 Q380 Seconds from origin time Procedure: 1. Transfer from individual station instrument response to STS2-HG through Q330. 2. Measure peak amplitude (PGV in m/sec) of Lg arrival on BHZ component.

  9. HOPS, WDC and COR would have clipped with High-Gain STS2

  10. BHZ

  11. Recommendation: Based on the analysis of the three methods discussed above it is our recommendation that the standard-gain STS2 is more appropriate for routine earthquake monitoring in the US. Method 1 indicates that if the high-gain configuration is used at all ANSS backbone stations, all stations can be expected to clip given the local ground shaking levels expected in the next 30 years. Methods 2 and 3 indicate that an M6 will clip stations within approximately 875km of the epicenter. The high-gain STS2 may be more useful for recording teleseismic events on a global scale but not suitable for on-scale recording of moderate to large earthquakes at regional distances within the continental US.

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